Is there a way to use a MIDI controller as a sensor in the Game Engine?
There is a script extension, Blender HID Mapper, that does the following:


  Create a HID Mapping script similar to the provided template and run it. Then you can control blender with your MIDI device like specified in your mapping.


Just install the most recent version of the script from this site, then write your mapping as you desire. The first link above also has a sample mapping script that you can work off of.